Paint compositions which contain (A) from 5 to 30 wt % of hydroxyl group containing lactone modified oligomer of weight average molecular weight not more than 1000 and of which the hydroxyl group value is from 200 to 800 where a lactone has been used, (B) from 5 to 50 wt % of hydroxyl group containing resin of weight average molecular weight from 1000 to 6000 and of which the hydroxyl group value is from 50 to 200, (C) from 30 to 70 wt % of polyisocyanate compound and (D) from 3 to 30 wt % of melamine resin are known as paint compositions with which the acid rain resistance and scratching resistance are excellent (for example, see Patent Citation 1). However, these paint compositions involve the use of low molecular weight hydroxyl group containing lactone oligomers and so the glass transition point (Tg) of the paint film is inevitably low and there is a disadvantage in that the staining resistance and weather resistance are reduced.
Furthermore, a technique in which (A) vinyl copolymer which has hydroxyl groups and carboxyl groups with a hydroxyl group value of from 20 to 200 mgKOH/g and an acid value of from 5 to 50 mgKOH/g and (B) blocked isocyanate compound where isocyanate compounds comprising (b1) aliphatic polyisocyanate compound which has three or more isocyanate groups in one molecule and (b2) aromatic and/or alicyclic polyisocyanate compound which has three or more isocyanate groups in one molecule in the ratio by mass of solid fractions from 20/80 to 80/20 has been blocked with a blocking agent comprising (b3) heterocyclic compound and (b4) active methylene compound in a mol ratio of from 50/50 to 90/10 are included and the mol ratio of the hydroxyl groups of the (A) component and the blocked isocyanate groups of the (B) component is from 1/2 to 2/1 in which a blocked isocyanate compound is used is known for thermosetting paint compositions which have excellent acid resistance, scratching resistance and durability (for example, see Patent Citation 2). However, with these paint compositions the un-blocking reaction of the heterocyclic compound selected from among 3,5-dimethylpyrazole or 1,2,4-triazole is slower than the ester-exchange reaction with the active methylene compound and so there is a weakness in that the hardenability at low temperature is reduced.
Furthermore, coating compositions which include one or more hydroxyl group containing acrylic resin selected from among the hard acrylic resins which have a glass transition temperature (Tg) of from 20° C. to 70° C. and the flexible acrylic resins which have a glass transition temperature (Tg) of from −100° C. to −10° C. where resins which have different glass transition temperatures are combined and one or more di(alkoxy)dicarboxylic acid ester end-capped polyisocyanate crosslinking agent are known as clear-coats which have excellent acid resistance and scratching resistance (for example see Patent Citation 3). However, since di(alkoxy)alkyl dicarboxylic acid ester end-capped polyisocyanate crosslinking agents are used there is a weakness in that the hardening properties and resistance to chemical attack are inadequate.
Furthermore, hardening resin compositions which have acrylic resins which contain hydroxyalkyl groups which have from 4 to 9 carbon atoms and polyfunctional isocyanate compounds as essential components, in which the soft segments make up from 25 to 50 mass % of the total resin solid fraction and which are prepared in such a way that the proportion of soft segment parts originating from lactone-containing monomer with respect to the total solid fraction of the hydroxyl group containing acrylic resin and the polyfunctional isocyanate compound is not more than 8 mass % are known as resin compositions for clear-paint purposes which have excellent scratching resistance and acid resistance (for example, see Patent Citation 4). However, acrylic resins which have a low resin Tg are used and so there is a weakness in that the hardness and resistance to chemical attack of the paint film are reduced.
[Patent Citation 1]
    Japanese unexamined patent application laid open 2002-105397[Patent Citation 2]    Japanese unexamined patent application laid open 2005-126649[Patent Citation 3]    Japanese unexamined patent application laid open 2005-179662[Patent Citation 4]    Japanese unexamined patent application laid open 2006-176632
Furthermore, EP 1 240 210 B1 discloses an ultra high coating composition. The composition comprises a polyol blend of a liquid acrylic polyol and a resinous polyol. The polyol blend is crosslinked with a multifunctional isocyanate, melamine, or silane to form a variety of coatings. The coating composition has a significantly higher solid % than that is formulated from a single resin that has similar composition of the combination of the polyol blend. The coatings include less than about 35% by weight of an organic solvent. The liquid acrylic polyol has a hydroxyl number within the range of 20 mg KOH/g to 500 mg KOH/g and a glass transition temperature (Tg) within the range of −70° C. to 0°. The resinous polyol has a hydroxyl number within the range of 20 mg KOH/g to 500 mg KOH/g and a glass transition temperature (Tg) greater than 25° C. The use of 25 to 55 mass % of 4-hydroxybutyl(meth)acrylate in the liquid acrylic polyol is not disclosed.